1. Field of the Invention
The present invention relates to laser beam scanning apparatus, and more particularly to a laser beam scanning apparatus for use in laser beam printers, facsimile systems, etc.
2. Description of the Prior Art
Generally in laser beam scanning apparatus, a laser beam emitted from a semiconductor laser is made into a bundle of parallel rays by a collimator lens and thereafter converged by a cylindrical lens to a linear form on a reflecting surface of a polygonal mirror. Upon deflection at the mirror reflecting surface, the laser beam is concentrated on a beam receiving surface by an imaging optical system. The spot of laser beam on the beam receiving surface moves with the rotation of the polygonal mirror, thereby forming a scanning line. In the following description, the direction in which this scanning line extends will be referred to as the "main scanning direction," and a direction orthogonal to the scanning line on the beam receiving surface as the "sub-scanning direction." The beam receiving surface is moved in the sub-scanning direction and exposed to the laser beam two-dimensionally with this movement. The reflecting surface of the polygonal mirror and the beam receiving surface are optically conjugate with respect to the sub-scanning direction. This precludes the deviation of the scanning line due to an inclination of the reflecting surface of the polygonal mirror.
On the other hand, Japanese Laid-Open Patent Application No. 61-28919 discloses a laser beam scanning apparatus having two laser beam generating sources. With the disclosed laser beam apparatus, two laser beams are incident on the reflecting surface of a polygonal mirror each at a predetermined angle in the sub-scanning direction.
However, when the distance from the axis of rotation of the polygonal mirror to its respective reflecting surfaces involves variations in the case where a laser beam is obliquely incident on the reflecting surfaces of the polygonal mirror in the sub-scanning direction, the spot of the laser beam on the beam receiving surface deviates in the sub-scanning direction, consequently giving rise to exposure irregularities periodically.